Method for manufacturing metal foam

ABSTRACT

A highly-productive method for manufacturing a metal foam, capable of easily manufacturing a molded article having a desired shape is provided. A method for manufacturing a metal foam includes dissolving hydrogen in a mixture containing a molten metal and a thickener, and thereby manufacturing a precursor in which an amount of solid-soluted hydrogen in the metal is saturated, charging the precursor into a mold, and solidifying the precursor charged into the mold under a reduced-pressure atmosphere, or solidifying the precursor charged into the mold and then heating the solidified precursor under a reduced-pressure atmosphere.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese patent application No. 2021-019331, filed on Feb. 9, 2021, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to a method for manufacturing a metal foam.

Metal foams which are porous materials made of a metal or an alloy and have a number of pores inside thereof are known. Since metal foams have an excellent impact energy absorption property and a sound deadening property, and have a light weight, they are used as multi-functional materials in a variety of fields. However, because of their high material costs and complicated manufacturing processes, their manufacturing costs are high, so it has been desired to reduce these costs.

International Patent Publication No. WO2010/106883 discloses a method for manufacturing a precursor of a metal foam and a method for manufacturing a metal foam, which have the following features. The manufacturing method disclosed in International Patent Publication No. WO2010/106883 makes it possible to easily manufacture a precursor of a metal form and/or a metal foam without using an expensive foaming agent powder. This manufacturing method improves the sphericity of pores in the metal foam and increases the porosity of the metal foam by adding alumina when friction stir processing (FSP) is performed. Firstly, a die-cast molded article containing a gas inside thereof is manufactured by a die-casting method. Next, a precursor of a metal foam is manufactured by uniformly dispersing the gas and pore-forming nuclei contained inside the die-cast molded article throughout the die-cast molded article by the FSP. Further, a metal foam is manufactured by heat-treating the precursor thereof in which the precursor is heated to a temperature close to its melting point, and thereby foaming the precursor.

SUMMARY

However, the manufacturing process of the manufacturing method disclosed in International Patent Publication No. WO2010/106883 is complicated. Further, it requires a die-casting apparatus or the like, so that the manufacturing equipment becomes larger, thus increasing the manufacturing cost. Therefore, the manufacturing method disclosed in International Patent Publication No. WO2010/106883 has a problem that the productivity is low.

The present disclosure has been made to solve the above-described problem, and an object thereof is to provide a highly-productive method for manufacturing a metal foam, capable of easily manufacturing a molded article having a desired shape.

A first exemplary aspect is a method for manufacturing a metal foam including: dissolving hydrogen in a mixture containing a molten metal and a thickener, and thereby manufacturing a precursor in which an amount of solid-soluted hydrogen in the metal is saturated; charging the precursor into a mold; and solidifying the precursor charged into the mold under a reduced-pressure atmosphere, or solidifying the precursor charged into the mold and then heating the solidified precursor under a reduced-pressure atmosphere.

According to the present disclosure, it is possible to provide a highly-productive method for manufacturing a metal foam, capable of easily manufacturing a molded article having a desired shape.

The above and other objects, features and advantages of the present disclosure will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a flowchart showing a method for manufacturing a metal foam according to a first embodiment;

FIG. 2 is a schematic diagram showing a precursor manufacturing process and a precursor charging process in the method for manufacturing a metal foam according to first embodiment;

FIG. 3 is a schematic diagram showing an example of a solidifying process in the method for manufacturing a metal foam according to first embodiment; and

FIG. 4 is a schematic diagram showing another example of the solidifying process in the method for manufacturing a metal foam according to first embodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

An embodiment according to the present disclosure will be described hereinafter with reference to the drawings. However, the present disclosure is not limited to the below-shown embodiment. Further, for clarifying the explanation, the following description and the drawings are simplified as appropriate.

An overview of a method for manufacturing a metal foam according to a first embodiment will be described with reference to FIG. 1. FIG. 1 is a flowchart showing a method for manufacturing a metal foam according to the first embodiment. As shown in FIG. 1, the method for manufacturing a metal foam according to this embodiment includes steps S1 to S3 described below.

In a precursor manufacturing process in the step S1, hydrogen is dissolved in a mixture M2 containing a molten metal M1 and a thickener T, and by doing so, a precursor M3 in which the amount of solid-soluted hydrogen in the metal is saturated is manufactured. In a charging process in the step S2, the precursor M3 is charged (e.g., injected or poured) into a mold 1. In a solidifying process in the step S3, the precursor M3, which has been charged into the mold 1, is solidified under a reduced-pressure atmosphere.

Each of the above-described processes will be described in detail with reference to FIGS. 2 and 3. FIG. 2 is a schematic diagram showing the precursor manufacturing process and the precursor charging process in the method for manufacturing a metal foam according to the first embodiment. FIG. 3 is a schematic diagram showing an example of the solidifying process in the method for manufacturing a metal foam according to the first embodiment.

As shown by a stage S1-1 in FIG. 2, in the precursor manufacturing process in the step S1, firstly, a metal, which is used as a raw material for a metal foam, and a thickener T are prepared. As the metal used as the raw material for a metal foam, a single metal element or an alloy can be used. Examples of such metals and alloys include aluminum, magnesium, titanium, iron, zinc, copper, aluminum alloys, magnesium alloys, titanium alloys, steel materials, zinc alloys, and copper alloys.

The metal used as the raw material (hereinafter also referred to as the raw-material metal) is melted in a stirring container 10. Alternatively, a metal that has been melted in advance is charged into the stirring container 10. There is no particular limitation on the form (e.g., the phase or the like) of the raw-material metal, so a bulk material that can be obtained at a low cost can be used as the raw material.

When the molten metal M1 is formed, the raw-material metal is heated so that its temperature falls within an appropriate temperature range equal to or higher than the melting point according to the element or the composition of the metal. For example, in the case where the raw-material metal is aluminum or an alloy containing aluminum as its main component, the temperature should be within a range of 550 to 800° C., preferably within a range of 650 to 700° C. In the case where the metal is magnesium or an alloy containing magnesium as its main component, the temperature should be in a range of 550 to 800° C. In the case where the metal is zinc or an alloy containing zinc as its main component, the temperature should be within a range of 300 to 550° C. In the case where the metal is copper or an alloy containing copper as its the main component, the temperature should be within a range of 900 to 1,200° C.

As for the thickener T, for example, one or a plurality of types of powders selected from metal powders such as a calcium powder and a magnesium powder, metal oxide powders such as an alumina powder and a magnesia powder, ceramic powders such as a silicon carbide powder and a silicon dioxide powder can be used. The thickener T increases the viscosity of the molten metal M1, and is preferably one that is chemically stable in the molten metal M1.

By thickening the molten metal M1 within an appropriate range by using the thickener T, the coarsening of pores during the foaming process can be suppressed. Further, the thickened molten metal M1 prevents the gas forming the pores from being released to the outside of the molten metal M1, and retains the gas in the molten metal M1 and thereby keeps closed pores. That is, by adjusting the viscosity of the molten metal M1, it is possible to control the form (the sphericity, the size, etc.) of the pores formed inside the molten metal M1, so that the pores are stabilized.

The thickener T is added in the molten metal M1 contained in the stirring container 10, and the molten metal M1 is sufficiently stirred under the atmosphere by using, for example, a stirring blade. As a result, a mixture M2 in which the thickener T is uniformly dispersed in the molten metal M1 is obtained. Regarding the procedure for obtaining the mixture M2, the heating and the stirring may be performed after the raw-material metal and the thickener T are mixed. If necessary, the thickener T may be uniformly dispersed even further in the molten metal M1 by applying ultrasonic waves to the mixture M2.

Next, as shown by a stage S1-2 in FIG. 2, hydrogen is dissolved in the metal. Examples of the method for dissolving hydrogen in the metal include a method in which the tip of a feeding tube 20 is inserted into the mixture M2, which has been obtained as described above, and a water vapor V, which serves as a hydrogen source, is blown into the mixture M2 through the feeding tube 20. Another conceivable method is a method in which the mixture M2 is left undisturbed under the condition of a high water-vapor partial pressure (under a high humidity). Alternatively, a hydrogen gas may be blown into the mixture M2 through the feeding tube 20.

The method for dissolving hydrogen in the metal is not limited to the above-described methods as long as the concentration of hydrogen in the molten metal M1 can be increased. The water vapor V is added in such an amount that the amount of hydrogen solid-soluted in the metal is supersaturated. If the amount of hydrogen solid-soluted in the metal is insufficient, the pores will not sufficiently grow during the foaming process.

Next, as shown by a stage S2 in FIG. 2, in the precursor charging process in the step S2, the above-described precursor M3 is charged (e.g., injected or poured) into a mold 1 whose shape conforms to that of the article to be produced. It is sufficient if the mold 1 has a required heat resistance and required durability according to the heating condition and the pressure-reducing condition in the solidifying process in the step S3. As for the material of the mold 1, stainless steel having an excellent heat resistance and excellent durability, heat-resistant steel having such properties, or the like can be used. Further, the mold 1 may also be used as the stirring container 10 as long as the material can be sufficiently stirred and the required amount of hydrogen can be solid-soluted into the mixture M2. In this case, there is no need to transfer (e.g., pour or charge) the molten metal from one container to another between the processes, so it is more efficient. The amount of the precursor M3 charged into the mold 1 is adjusted based on the degree of the foaming.

Next, as shown in FIG. 3, in the solidifying process in the step S3, the precursor M3 charged into the mold 1 is solidified under a reduced-pressure atmosphere. To achieve a reduced-pressure atmosphere, for example, a vacuum apparatus 30 a including a vacuum chamber 31, a vacuum pump 32, and a vacuum valve 33 is used. The precursor M3 charged into the mold 1 is placed inside the vacuum chamber 31, and the pressure in the vacuum chamber 31 is reduced by operating the vacuum pump 32. Note that the pressure-reducing is preferably started at a temperature at which the metal becomes a solid solution state. The level of vacuum inside the vacuum chamber 31 is maintained, for example, in an intermediate vacuum state (i.e., at a pressure of 10² Pa to 10⁻¹ Pa). In the case where the inside (i.e., the internal space) of the mold 1 can be brought into a reduced-pressure state by making the inside of the mold 1 a closed space, the vacuum pump 32 may be connected to the mold 1.

During this solidifying process, the hydrogen, which has been solid-soluted in the metal, is released as the temperature of the molten metal M1 decreases. The hydrogen is released as a gas. Further, the released gas expands as the pressure decreases, so that a larger number of pores are generated inside the molten metal M1. Regarding the release of hydrogen as a gas, in the case where alumina, magnesia, or the like is added in the mixture, the gas is more likely to be released because such an inclusion (i.e., an additive) acts as pore-forming nuclei. Then, the gas in the molten metal M1 transforms into bubbles due to the pressure difference caused by the reduction in pressure inside the vacuum chamber 31, so that the molten metal M1 solidifies with pores entrapped inside thereof. In this way, a metal foam including a large number of pores inside thereof can be manufactured.

As for the solidifying process in the step S3, instead of using the above-described method, a method in which the precursor M3 charged into the mold 1 is first solidified and then heated under a reduced-pressure atmosphere can also be used. Therefore, another example of the solidifying process will be explained with reference to FIG. 4. FIG. 4 is a schematic diagram showing another example of the solidifying process in the method for manufacturing a metal foam according to the first embodiment.

In the other aspect of the solidifying process (Step S3), firstly, a solidified metal body M4 is formed by quenching and solidifying the precursor M3 obtained by the processes in the steps S1 and S2 as shown in a stage S4-1 in FIG. 4. The method for solidifying the precursor M3 is, for example, a method in which the precursor M3, together with the mold 1 containing it, is submerged in a water tank 40 containing a refrigerant R. As a result, a solidified metal body M4, which is formed by quenching and solidifying the precursor M3, is obtained. As for the refrigerant R, water, liquid nitrogen, or the like can be used. However, the method for solidifying the precursor M3 is not limited to the above-described method.

Next, as shown in a stage S4-2 in FIG. 4, a vacuum heating process is performed on the obtained solidified metal body M4. The vacuum heating process is performed, for example, by using a vacuum heating apparatus 30 b including a vacuum chamber 31, a vacuum pump 32, a vacuum valve 33, heating means 34 such as a heater, and cooling means using a nitrogen gas (not shown). The obtained solidified metal body M4 is placed inside the vacuum chamber 31, and the inside (i.e., the internal space) of the vacuum chamber 31 is heated by using the heating means 34. Note that the obtained solidified metal body M4 is heated to a temperature at which the metal contained in the solidified metal body M4 becomes a solid solution state. As a result, hydrogen contained in the solidified metal body M4 is released as a gas.

Further, after stopping the heating, the pressure inside the vacuum chamber 31 is reduced by operating the vacuum pump 32, and the inside of the vacuum chamber 31 is cooled by using the cooling means at the same time. A metal foam is obtained by cooling and solidifying the heated solidified metal body M4 under a reduced-pressure atmosphere. Under the reduced-pressure atmosphere and in the cooled state, the released gas is fixed in an expanded state. In this way, it is possible to manufacture a metal foam including a large number of pores inside thereof.

As described above, according to the method in which the precursor M3 is first solidified and then a vacuum heating process is performed thereon, the temperature of the object to be processed (i.e., the solidified metal body M4) placed inside the vacuum chamber 31 can be easily controlled, so that the productivity is improved. Further, since the accuracy of the control of the temperature is improved, the quality of the metal foam is improved.

Next, the present disclosure will be described in a more detailed manner based on Examples 1 and 2. However, the present disclosure is not limited to these examples.

EXAMPLE 1

In this example, aluminum was used as the raw-material metal. Molten aluminum was obtained by heating the aluminum to 650 to 700° C. in a stirring container 10. Then, 1.5 wt. % of granular calcium T1 and 1.5 wt. % of a alumina powder T2 were added in this molten aluminum, and the molten aluminum was stirred at 500 to 1000 rpm for 20 minutes. As a result, a mixture M2 containing the molten aluminum, the calcium, and the alumina was obtained.

Note that the particle diameter of the used alumina powder T2 was 50 μm. As the alumina powder T2, an oxide that is formed when a metal that is easily oxidized, such as aluminum, is kept in a molten state in the atmosphere, and the metal reacts with oxygen (when the metal is aluminum, it becomes alumina) can be used. When such an unnecessary oxide (so-called slag) is used as the alumina powder T2, the manufacturing cost of metal foams can be further reduced.

The tip of a feeding tube 20 was placed in the above-described mixture M2, and a water vapor V was added in (i.e., blown into) the mixture M2 through the feeding tube 20. The water vapor V was added as water (H₂O) in an amount of 3 to 4 mol/kgAl. As a result, a precursor M3 in which the amount of the solid-soluted hydrogen in the aluminum was saturated was manufactured. The precursor M3 was manufactured in the atmosphere.

The manufactured precursor M3 was transferred from the stirring container to a mold 1 which was formed so as to have relatively thin walls. In this embodiment, a mold having a cup-like shape having an opened top and a closed bottom was used as the mold 1.

Next, the precursor M3, together with the mold 1 containing it, was placed inside the vacuum chamber 31 of the vacuum apparatus 30 a. Then, the pressure inside the vacuum chamber 31 was reduced from the atmospheric pressure to about 10 Pa in a state in which the temperature of the precursor M3 is around 500° C., and the precursor M3 was cooled and solidified under the reduced-pressure atmosphere. As a result, hydrogen that had been solid-soluted in the aluminum was released as a gas.

Further, the gas was expanded due to the pressure difference caused by the reduction in pressure, and the molten aluminum was solidified in a state in which a large number of pores were formed inside the molten aluminum. Then, the solidified metal foam molded article was removed from the mold 1, so that porous aluminum having a density of about 0.9 g/cc and a porosity of about 65% was obtained.

Note that the generation of hydrogen by the reaction of the aluminum and the water vapor V (water) is expressed by the below-shown Expression (1).

2Al+3H₂O—>AlO₃+6H   Expression (1)

EXAMPLE 2

This example was carried out according to the solidifying process shown in FIG. 4. Firstly, a precursor M3 was manufactured in a method similar to that in the Example 1, and charged into a mold 1. After that, a solidified metal body M4 was obtained by submerging the precursor M3 charged into the mold 1 in a water tank 40 in which water was contained as a refrigerant R. This solidified metal body M4 contained at least aluminum, a calcium oxide, alumina, and hydrogen.

Next, a vacuum heating process was performed on the obtained solidified metal body M4 by using a vacuum heating apparatus 30 b. In the vacuum heating process, the solidified metal body M4 charged into the mold 1 was placed inside the vacuum chamber 31, and the solidified metal body M4 was heated to around 500° C. After that, the heating was stopped. Then, the pressure inside the vacuum chamber 31 was reduced from the atmospheric pressure to about 10 Pa, and the heated solidified metal body M4 was quenched at the same time. As described above, the vacuum heating process was performed on the solidified metal body M4, and the cooled and solidified metal foam molded article was removed from the mold 1, so that porous aluminum similar to that in the Example 1 was obtained.

It should be noted that, for example, a die-casting apparatus is required in the manufacturing method disclosed in International Patent Publication No. WO2010/106883 in order to manufacture a metal foam, so that the manufacturing facility becomes larger. Further, it is difficult to form a metal foam having a complicated shape by the method using a die-casting apparatus. Further, in the manufacturing method disclosed in International Patent Publication No. WO2010/106883, the process of obtaining a precursor of a metal foam by repeatedly performing friction stirring on a die-cast molded article is complicated, so that its productivity is low.

In contrast, the method for manufacturing a metal foam according to this embodiment does not require a die-casting apparatus and its manufacturing process is simple. Further, as for the mold 1 used to mold a metal foam, a mold having walls that are thinner than those of the die-casting mold used in the die-casting apparatus can be used. Therefore, the cost for the method for manufacturing a metal foam is low, and therefore its productivity is high. Further, since it is possible to use a mold having a complicated shape, a metal foam manufactured by using the method for manufacturing a metal foam according to this embodiment has high flexibility in regard to its shape.

Further, as another method for manufacturing a metal foam, there is direct foaming in melt method using a hydride such as titanium hydride and zirconium hydride as a foaming agent. However, since such a foaming agent is expensive, it increases the manufacturing cost of metal foams. Further, in the case where a foaming agent having a low thermal decomposition temperature is used, the material used as the raw material is limited to those having relatively low melting points.

In contrast, in the method for manufacturing a metal foam according to this embodiment, an inexpensive material such as a water vapor can be used as the hydrogen source for forming pores. According to the above-described features, it is possible to manufacture a metal foam without using an expensive foaming agent, and thereby to reduce the manufacturing cost of metal foams. Further, the restrictions on the material of the metal which are imposed because of the thermal decomposition temperature of the foaming agent are relaxed.

Therefore, according to the method for manufacturing a metal foam according to this embodiment, it is possible to easily manufacture a molded article having a desired shape by using a simple apparatus(es) and a simple material(s).

From the disclosure thus described, it will be obvious that the embodiments of the disclosure may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 

What is claimed is:
 1. A method for manufacturing a metal foam comprising: dissolving hydrogen in a mixture containing a molten metal and a thickener, and thereby manufacturing a precursor in which an amount of solid-soluted hydrogen in the metal is saturated; charging the precursor into a mold; and solidifying the precursor charged into the mold under a reduced-pressure atmosphere, or solidifying the precursor charged into the mold and then heating the solidified precursor under a reduced-pressure atmosphere. 